[FFmpeg-devel] ALT_BITSTREAM_READER vs. A32 on ARMv4

Mike . giac2000
Sat Dec 27 02:27:29 CET 2008

> To: ffmpeg-devel at mplayerhq.hu
> From: mans at mansr.com
> Date: Fri, 26 Dec 2008 23:57:55 +0000
> Subject: Re: [FFmpeg-devel] ALT_BITSTREAM_READER vs. A32 on ARMv4
> "Mike ." <giac2000 at hotmail.com> writes:
> > Hi.
> >
> > I'm decoding wma on ARMv4 using the ALT_BITSTREAM_READER.  The list
> > indicates that the A32_BITSTREAM_READER should be faster, however in
> > my testing its actually a bit slower.  Is there anything I'm missing
> > (I simply forced the definition in bitstream.h to one or the other and
> > benchmarked)?  Perhaps this is normal for ARM7TDMI?
> >
> > Also, whats different about the two readers?  I've started digging
> > through them but I'm not really sure why they do things differently.
> Using 32-bit aligned loads is often faster than several smaller loads
> on architectures that do not support unaligned accesses.  Sometimes
> extra processing overhead required to take advantage of this kills the
> improvement.  The bitstream readers are unfortunately rather sensitive
> to specifics of the CPU, so benchmarks are usually the only accurate
> way to tell which is faster.
> Please try all three bitstream readers and report your results.

Thanks for the advice.

My results (percent real time for a 192k WMAv2 track):




(Tested on an 80MHz Portal Player PP5024)


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